1. Technical Field of the Invention
This invention relates to means for the drying of microelectronic wafer structures so as to avoid surface tension effects, and more particularly to methods and apparatus for drying silicon wafer microstructures with supercritical fluids.
2. Background Art
Working to find improvements to the controlled release of microstructures without subsequent sticking of these structures to the substrate, researchers at the University of California at Berkeley have developed a process for drying silicon wafers in a supercritical fluid environment. In this state, there is no liquid/vapor interface to contribute to the surface tension effects that cause long, thin microstructures in the order of 50 micrometers and up, to collapse and stick to the substrate during the drying process. The supercritical fluid of choice was CO.sub.2, carbon dioxide, due to it's temperature and pressure thresholds of 31.1 degrees centigrade and 1073 pounds per square inch over atmosphere.
Using the laboratory method, a silicon wafer containing a pattern of microelectronic structure, having been fabricated in the conventional manner, is arranged with a horizontal orientation in a cavity, submerged in methanol. The cavity is sealed, and a through-flow of liquid carbon dioxide (CO.sub.2) is introduced for about 15 minutes. The methanol is rapidly absorbed into the liquid carbon dioxide and carried out of the cavity. When the cavity has been entirely purged of methanol and is completely filled with pure liquid carbon dioxide, heat is applied uniformly for several minutes, causing the carbon dioxide to transition to it's supercritical phase. It is at this point that the benefit of the process is realized, as no liquid/vapor interface occurs during this transition. The CO.sub.2 is then slowly vented to atmosphere, to avoid turbulence within the cavity.
A vessel that is openable at or near the plane of maximum cross section area of interior volume, and when closed is subjected to greatly elevated temperature and pressure, must be of substantial construction, with a locking mechanism adequate to safely sustain the total pressure applied. In the university laboratory set up, a circumferential pattern of 8 bolts is used to secure the lid or top to the base of the vessel, to contain the high pressure. The subject wafer is placed within the base cavity, the lid placed in position, the fasteners applied manually to secure the lid, heat applied to the vessel by external heaters, and ports in the vessel used to admit and remove the fluids of the process.
There are several obvious problems with the laboratory set up that must be addressed in order to make this process sufficiently cost-effective and efficient for use in a production environment. The device is not suitable for integration into a production line with automated means for inserting and removing wafers; the closing mechanism is manual and too slow; and the serially administered steps of the process are manually accomplished and too slow. The device is also lacking the safeguards required by industrial standards and regulations for production requirements.